1. Field of the Invention
The invention relates to complex pigment compositions for the coating of paper, containing a high concentration of one or more pigments having a viscosity which is low and is stable with time.
2. Discussion of the Background
Those skilled in the art have for a long time had available pigment compositions for the coating of paper. These compositions contain pigments, ordinarily of a known type such as kaolin, comprised of more or less substantial quantities of clays, an aqueous phase in which the pigments are dispersed, a binding agent of natural origin which may be water-dispersed or water-soluble (e.g. starch, casein, or carboxymethylcellulose), and/or a synthetic binding agent (e.g. styrene/butadiene emulsions, styrene/acrylate emulsions, or vinyl copolymer emulsions), and a dispersing agent (e.g. a polyphosphate).
There has been rapid development in paper coating techniques due to advancement of the relevant techniques and improvement in coating speeds. As a result, there has come about a requirement that pigment compositions for coating paper must also contain the minimum possible amount of water, in order to reduce the heat energy consumed in drying, and in order to attain the most favorable rheological characteristics for the coating operation. These characteristics are a low viscosity and high velocity gradient, so that coating can be accomplished very rapidly; or to enable low shear stress via an always-low viscosity, for easy handling of the coating composition, particularly during sieving operations prior to the coating operation.
Pigment compositions for the coating of paper have themselves undergone major evolution in recent years in response to technical progress (major increases in coating speed) and to needs relating to productivity, quality, and cost. Beside the reduction of the amount of water, improvements have come in the form of increased concentration of the pigment materials, and particularly in the use of alkali metal or ammonium polyacrylates as dispersants. This enables pigments to be maintained in a dispersed state, after being mechanically comminuted, so that the coating composition is sufficiently fluid.
In this connection, French Patent No. 2,185,721 describes pigment compositions for coating paper, wherein the dispersant is chosen from the acrylic acid polymers completely neutralized by an alkaline sodium hydroxide solution. However, such dispersants have major disadvantages in that they have been found to be difficult to use in certain pigment compositions currently used for coating paper. In particular, they have been shown to be sensitive to pH variation in the aqueous phase of the pigment dispersion in the ionic environment of the compositions, resulting in rapid and irreversible increases in viscosity, and in some cases bulk setting of the coating composition.
The pigment compositions for the coating of paper which are presently available have advanced pigment formulations. They increasingly often employ mixtures of pigments which are no longer comprised solely of kaolin or of kaolin and clays, but are comprised of kaolin and/or pigmented calcium carbonate and/or talc and/or titanium dioxide, whereby the ionic state of the aqueous phase is changed. Accordingly, the abovementioned increases in viscosity (and in certain cases the abovementioned setting) occur during preparation of these pigment compositions and during mixing of the pigment compositions prior to their being applied to coat paper and/or being stored. In particular, these adverse phenomena occur in the case of mixtures of pigments.
Thus, the known dispersants as employed are incapable of eliminating the problems caused by the presence of mixtures of pigments which are more or less compatible with each other, in the pigment compositions for coating paper. Nonetheless, these mixtures are required in the compositions for reasons of improving the quality of the paper after coating.
The present Applicant, in another area of technology, had previously encountered the phenomenon of viscosity increase during crushing and grinding of mineral materials in aqueous medium, and had successfully proposed the use of a "comminution agent", a ethylenic polymer. This enabled the concentration of mineral matter in the suspension fed to the crushing and grinding operation to be increased, while maintaining a low viscosity which was stable with time during the comminution and the prolonged (1 month) storage of the suspensions of comminuted materials.
In this connection, French Patent No. 2,531,444 describes a comminution agent comprised of a polymer and/or copolymer of ethylenic acids, wherein the acid functions are partially neutralized by at least one neutralizing agent having at least one monovalent function, wherewith the degree of neutralization may be between 0.40 and 0.96, preferably between 0.50 and 0.75. Here the nature of the neutralizing cation is not essential. Rather, the important characteristic is the amount of free acidity following the neutralization.
However, when this comminution agent is introduced into a pigment composition comprised of a high concentration of pigments (e.g. 70%) comprising a single pigment (e.g. kaolin) or a plurality of pigments (e.g. pigmented calcium carbonate and titanium dioxide) for coating paper, the comminution agent causes a substantial increase in the viscosity and indeed in certain extreme cases it causes the composition to set, even though one would expect just the opposite, i.e., a lowering of the viscosity, based on the behavior of the polymeric agent as a comminution agent.
French Patent No. 2,539,137 describes a comminution agent comprised of a polymer and/or copolymer of ethylenic acids, wherein the acid functions are completely and simultaneously neutralized by at least one neutralizing agent having a monovalent function and at least one other neutralizing agent having a polyvalent function. The monovalent-type neutralizing agent comprises between 40 and 95%, preferably between 60 and 90%, and the polyvalent-type neutralizing agent comprises between 60 and 5%, preferably between 40 and 10% of the total neutralizing agent.
However, when this comminution agent is employed as a dispersant in pigment compositions for coating of paper according to the prior art method, the compositions comprising a very high concentration of one pigment (e.g., kaolin, such as "Dinkie A lump", at a concentration of 68%) or of a mixture of a plurality of pigments (e.g., calclum carbonate and titanium dioxide), a rapid and irreversible change of the rheological characteristics of the compositions occurs. In particular a substantial increase in viscosity occurs, and in certain cases bulk setting, particularly in the case of a mixture of pigments, even if it is a mixture of two pigments with one present in a very small proportion.